Chain-Amplified Photochemical Fragmentation of N-Alkoxypyridinium Salts: Proposed Reaction of Alkoxyl Radicals with Pyridine Bases To Give Pyridinyl Radicals

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• 作者：Deepak Shukla ; Shashishekar P. Adiga ; Wendy G. Ahearn ; Joseph P. Dinnocenzo ; Samir Farid
• 刊名：The Journal of Organic Chemistry
• 出版年：2013
• 出版时间：March 1, 2013
• 年：2013
• 卷：78
• 期：5
• 页码：1955-1964
• 全文大小：436K
• 年卷期：v.78,no.5(March 1, 2013)
• ISSN：1520-6904

文摘

Photoinduced electron transfer to N-alkoxypyridiniums, which leads to N鈥揙 bond cleavage and alkoxyl radical formation, is highly chain amplified in the presence of a pyridine base such as lutidine. Density functional theory calculations support a mechanism in which the alkoxyl radicals react with lutidine via proton-coupled electron transfer (PCET) to produce lutidinyl radicals (BH^{鈥?/sup>). A strong electron donor, BH鈥?/sup> is proposed to reduce another alkoxypyridinium cation, leading to chain amplification, with quantum yields approaching 200. Kinetic data and calculations support the formation of a second, stronger reducing agent: a hydrogen-bonded complex between BH鈥?/sup> and another base molecule (BH鈥?/sup>路路路B). Global fitting of the quantum yield data for the reactions of four pyridinium salts (4-phenyl and 4-cyano with N-methoxy and N-ethoxy substituents) led to a consistent set of kinetic parameters. The chain nature of the reaction allowed rate constants to be determined from steady-state kinetics and independently determined chain-termination rate constants. The rate constant of the reaction of CH₃O鈥?/sup> with lutidine to form BH鈥?/sup>, k₁, is 6 脳 106 M鈥? s鈥?; that of CH₃CH₂O鈥?/sup> is 9 times larger. Reaction of CD₃O鈥?/sup> showed a deuterium isotope effect of 6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases k₁ by a factor of 400.}